A probe is a type of a transducer which includes a movable stylus for detecting contact with an object. Some probes provide an output proportional to stylus deflection while others simply make or break an electrical circuit when the stylus moves from its rest position upon contact with an object. The latter type of probes are generally referred to as touch probes. U.S. Pat. No. 4,451,987 to Cusack discloses an example of a touch probe.
Present day probe constructions often consist of a switch head that is either directly connected to an adapter or to an intermediate probe body containing signal transmission electronic circuitry, which, in turn, is connected to the adapter. When the probe is used in machining centers, the adapter is designed so that the probe can be used in the machine tool spindle in the same manner as the cutting tools. When the probes are to be used in turning centers, the probe body is often designed so that it can be inserted into slots in a turret or the like so that the probe can be likewise mounted in the same manner as other tools used in the machine.
The switch head of a probe generally consists of a movable member having a first set of contacts thereon that oppose a second set of fixed contacts. Some type of biasing arrangement is applied to the movable member for urging it toward the fixed contacts to maintain the first and second set of contacts in engagement when the stylus is in a rest position. The spring pressure within the probe causes the stylus to sit in a unique rest position and returns the stylus to this position following each deflection. It sometimes becomes necessary to adjust the spring pressure on the movable member in special circumstances, for example, when excessive machine vibration causes spurious readings or when the weight of the stylus is too great to be supported by the standard pressure.
One of the most common techniques for adjusting the stylus spring pressure requires the user to disconnect the switch head from its mounting (see, e.g. page 11 of "User's Handbook MP-3 Probe With 360 Degree Optical Transmission System" and page 32 of "User's Handbook LP-2 Probe System", both available from Renishaw Electrical Ltd.) These known approaches generally utilize an adjusting screw arrangement on the rear of the switch head to adjust the spring pressure. This procedure is obviously somewhat annoying and time consuming since it requires the user to disassemble the probe and then re-assemble it after making the adjustment. Other approaches have been suggested as, for example, disclosed in an Instruction Manual entitled "Telemetry Touch Probe Model 733 Boring Bar Probe" available from the Digital Techniques division of the Assignee of the present invention. In general, this technique compresses or relaxes the spring, depending upon the relative spacing between the switch head and a fixed annular receptacle on the probe body which are threadably coupled together. The Renishaw Model TP-4 probe also apparently uses some type of external rotatable ring construction on the probe to adjust spring pressure.